LAS involves directing laser energy at a sample of matter in order to disassociate its constituent parts and make them available to a spectrometer for processing. Operation of LAS systems and other laser assisted spectroscopy systems typically apply this energy to the sample while passing a fluid, typically an inert gas, over the sample to capture the disassociated species and carry them to a spectroscope for processing. Sampling and detecting constituent parts of a sample with mass or optical spectrometry using an inert gas flow is necessary since, for example, an inductively coupled plasma instrument depends upon a plasma torch to ionize the laser ablated material for subsequent processing. This plasma torch can only operate in an inert atmosphere since regular open atmosphere extinguishes the plasma torch. Another advantage to using inert gas flow for laser assisted spectroscopy is that certain inert gases are transparent to desired laser wavelengths whereas regular room atmosphere is not. In addition, inert atmospheres can prevent chemical changes to ablated materials that could take place in room atmosphere.
Commonly, LAS systems require opening their sample chambers to remove old samples and insert new samples. While this is happening, it is important to maintain the flow of inert gas to the spectrometer and prevent air from reaching the plasma torch and extinguishing it, among other reasons. For the same reasons, the sample chamber must be purged of air prior to connection to the spectrometer following opening and closing. Once the plasma torch is extinguished, the system must be restarted and recalibrated, taking time and expertise. In order to prevent room atmosphere from entering the instrument, care must be taken when the sample chamber is opened to insert a new sample. The problem of purging a sample chamber of room atmosphere following insertion of a new sample has been previously considered with varying results.
Laser assisted mass spectroscopy is described in U.S. Pat. No. 5,135,870 LASER ABLATION/IONIZATION AND MASS SPECTROSCOPIC ANALYSIS OF MASSIVE POLYMERS, inventors Peter Williams and Randall W. Nelson, Aug. 4, 1992. This patent describes using a laser to ablate a thin film of organic material in a vacuum and thereafter analyze it using a mass spectrometer. A more recent publication, US patent application No. 2009/0073586A1 ANALYTICAL LASER ABLATION OF SOLID SAMPLES FOR ICP, ICP-MS, AND FAG-MS ANALYSIS, inventors Robert C. Fry, Steven K. Hughes, Madeline J Arnold, and Michael R. Dyas, Mar. 19, 2009 describes in detail a radiation-hardened sample chamber design for a laser ablation system. A reference which discusses the issue of purging sample cells is U.S. Pat. No. 4,640,617 SPECTROMETERS HAVING PURGE RETENTION DURING SAMPLE LOADING, inventors Norman S. Hughes and Walter M. Doyle, Feb. 3, 1987. This patent discloses and claims a means for minimizing the amount of air introduced into the sample chamber during sample loading by using a spring-loaded plunger to seal the sample chamber while loading a sample. U.S. Pat. No. 5,177,561 PURGING OF OPTICAL SPECTROMETER ACCESSORIES, inventors Milan Milosevic and Nicolas J. Harrick, Jan. 5, 1993 discloses a means to minimize purging by separating the sample chamber atmosphere from the spectrometer atmosphere, thereby eliminating the need to purge the spectrometer when samples are changed.
These patents have considered issues associated with purging sample chambers, mainly by minimizing the amount of room atmosphere introduced into the sample chamber as a new sample is introduced but have not considered solutions which alter the fluid flow through the system as the sample chamber is opened and closed. FIGS. 1a-c show an example of a prior art solution to the problem of providing: 1. Gas bypass when the sample chamber is open; 2. Gas purge when the sample chamber is initially closed; and, 3. Restoring gas flow after the sample chamber is purged. In FIG. 1a, fluid flow 14 (represented by the arrows marked “IN”: and “OUT”) enters the system via fluid inlet 12. This fluid flow 14 then enters inlet valve 16, which is in the “input bypass” position, sending the fluid 14 through the bypass tube 22 to the fluid outlet 24. The outlet valve 20 is in the “output bypass/purge” position closing communication between the sample chamber 10 and the fluid outlet 24. In this position, the sample chamber door 11 can be opened to remove or insert samples without risking contamination of the instrument (not shown) attached to the fluid outlet 24. In FIG. 1b, the inlet valve 16 is set to the “purge/restore” position, sending fluid 14 from the fluid inlet 12 to the sample chamber 10 via the inlet tube 18 and then onto the outlet valve 20 via the outlet tube 28. The outlet valve 20 is set to the “bypass/purge” position, sending the fluid from the sample chamber to the vent 26, thereby purging the sample chamber 10. In this mode, the sample chamber door 11 is closed. In FIG. 1c, the inlet valve 16 is set to the “purge/restore” position, sending the fluid 14 from the fluid inlet 12 to the sample chamber 10 via the inlet tube 18. The outlet valve 20 is set to the “restore” position, sending fluid 14 from the sample chamber 10 to the fluid outlet 24 via the bypass tube 22 while the sample chamber door 11 is closed. This exemplary prior art solution involves adding valves or other mechanisms to the sample chamber and the input and output gas ports. These valves or mechanisms are then operated or opened and closed manually in specific sequences prior to the sample chamber being opened and closed in order to create the bypass, purge and restore functions. Providing these functions manually requires additional time to open and close valves between samples, thereby reducing system throughput. In addition, requiring such a sequence of steps each time a sample is introduced increases system complexity, increases system and maintenance cost, and makes mistakes in operation more likely.
Accordingly, there is a continuing need for a way to introduce samples to a sample chamber including gas bypass, purge and restored flow in a laser ablation mass spectroscopy system automatically as the sample chamber is opened and closed to obviate the need for slow and error prone manual processes.